Heat butter in a large saucepan over medium-high heat. Add the leek and cook, stirring often, until starting to turn golden, about 4 minutes.

Add the cauliflower, apples, broth, and 500 mL (2 cups) water. Bring to a boil; Reduce heat and simmer until apple and cauliflower are tender when pierced with a fork, about 15 minutes.

Using an immersion blender or food processor, pulse the cauliflower mixture until creamy and smooth, about 2 minutes. Return soup to saucepan and warm over medium-low heat until heated through. Divide soup among six bowls and garnish with parsley.